How do I create a smooth slope to join 2 planes of different elevation?
To create such a slope is fairly simple.
Create a row of verities at the top and bottom of the slope. Make sure that the rows have the same amount of verities. Then you can bridge the gap.

If you want to create a bulge/indent, add a few edge loops running down the slope. next using proportional editing o, drag the center edge loop up or down. Using these techniques you can mold you slope. I would recommend sculpting for fine tuning.